(a) Field of the Invention
The present invention relates to stretched polyethylene multilayer films suitable for use as, for example, packaging materials and sealant materials, more particularly, to stretched polyethylene multilayer films which have excellent low-temperature heat-sealability, high mechanical strength, including excellent pin hole resistance and excellent impact resistance, and good applicability to various processing machines, such as laminating machines, bag-making machines and automatic filling machines, and can be advantageously used particularly as various kinds of packaging films and sealant materials for various kinds of multilayer films.
(b) Description of the Related Art
Typical heat-shrinkable films hitherto known are stretched films of polyvinylchloride, polypropylene or polyethylene. Among these films, polyethylene heat-shrinkable films have been put practical use as various kinds of packaging materials, including laminating materials and bag-making materials, because of their heat-sealability and inexpensive prices.
Among various kinds of polyethylenes including copolymers, linear copolymers of ethylene and .alpha.-olefins, which are called linear low-density polyethylenes (LLDPE), have attracted special interest recently.
As compared with conventional high-pressure process low density polyethylenes, the ethylene-.alpha.-olefin copolymers (LLDPE), which are linear low-density polyethylenes, can be produced with lower energy costs, and, in use as films, exhibit excellent mechanical properties, including high tear strength and high impact resistance, and are as well excellent in packaging processing properties, such as resistance to impurities during sealing and hot-tacking properties. This is the reason why LLDPEs are particularly preferred as various kinds of packaging films, and why great hopes are entertained of LLDPEs as sealant materials for multilayer films.
However, the requirement for the properties of such packaging films and sealant materials has become strict yearly, and in particular, the problems in the heat-sealability at low temperatures and in breakage of film packages and the formation of pin holes during transport cannot be solved by films of the conventional property levels. That is, films for such use require, particularly, further improvements of heat-sealability at low temperatures and of pin hole resistance, with the conventional applicability to processing machines maintained.
In order to develop films satisfying such strict requirements for properties, there have been attempted various improvements of stretched polyethylene (multilayer) films made from LLDPEs. However, there has been proposed no technique relating to stretched multilayer sealants made from LLDPEs, and heat-shrinkable films have been the mainstream of the conventional films of this kind.
For example, in Japanese Patent Application Kokai Koho (Laid-open) No. 1-301251 is disclosed a biaxially stretched heat-shrinkable multilayer film of a sandwich-type layer structure that consists of an intermediate layer of an ethylene-.alpha.-olefin copolymer (LLDPE) having a relatively high density of 0.915 to 0.930 g/cm.sup.3 sandwiched between two outer layers of an ethylene-.alpha.-olefin copolymer (LLDPE) having a relatively low density of 0.870 to 0.915 g/cm.sup.3. Although the heat-shrinkable film is described therein as having good applicability to packaging machines and having excellent heat-sealability at low temperatures, its heat-sealability at low temperatures particularly is still insufficient for satisfying the above-described strict requirements for the properties. The means employed for the improvement of the properties of the heat-shrinkable film, including the heat-sealability at low temperatures, are lowering of the density of the materials (the use of LLDPEs) and lamination of LLDPEs having different densities. It should be noted that the melt indexes of the LLDPEs used in the intermediate layer and both the outer layers are ranging from 0.1 to 0.3 g/10 min and from 0.2 to 0.3 g/10 min, respectively, and are low and of approximately the same degree, and that there is no description of use of LLDPEs having high melt indexes.
In Japanese Patent Application Kokai Koho (Laid-open) No. 1-304938 is disclosed a heat-shrinkable multilayer film wherein the materials of the intermediate layer and the outer layers of the film disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 1-301251 are simply exchanged with each other. The heat-shrinkable film, however, also involves the problem of insufficient heat-sealability at low temperatures. Further, the exchange of the materials between the intermediate layer and the outer layers does not make the heat-shrinkable film different from the above-described one in the points that selection of the densities of the materials is employed to improve the heat-sealability at low temperatures, that no LLDPE of high melt index is used and that the intermediate layer and the outer layers are made of LLDPEs having approximately the same degree of low melt indexes.
Further, the main object of the biaxial stretching of the multilayered films employed for the production of the above-described heat-shrinkable films is to endow the films with heat-shrinkability. Although relieving the heat-shrinkability endowed to the films by such stretching and providing non-shrinkability need a heat set by a heat-treatment near melting points, neither Japanese Patent Application Kokai Koho (Laid-open) discloses such heat set. These conventional stretched heat-shrinkable polyethylene multilayer films, therefore, are no better than heat-shrinkable films as their names impart, and involve a problem in that their uses are limited due to the inapplicability to the fields requiring non-shrinkable films.
There are other known polyethylene films, which are not multilayer films but ones heat-treated after a biaxial stretching. For example, in Japanese Patent Application Kokai Koho (Laid-open) No. 3-39332 is proposed to subject a mono-layer film of a blend of a LLDPE and a HDPE (high density polyethylene) to a sequentially biaxial stretching using a tenter method and to heat-treat the stretched mono-layer film at 80.degree. to 110.degree. C. The biaxially stretched resin-blend mono-layer film, however, is disadvantageous to packaging films because it requires high heat-sealing temperatures and are inferior in transparency. Further, the stretched polyethylene blend mono-layer film is shrinkable even after the heat-treatment, and the object thereof also is the use as a shrinkable film. That is, the film is also disadvantageous in that the improvement of the heat stability of the film by heat set is insufficient so that the applicable fields are limited in common with the above-described conventional heat-shrinkable films.